In the fabrication process for semiconductor devices, numerous processing steps, i.e., as many as several hundred, must be performed on a semiconducting wafer in order to form the devices. Among the numerous processes, a wafer is frequently positioned on a rotating wafer platform, or a vacuum chuck, for carrying out the specific process. For instance, some of processes are the photoresist coating process, the photoresist developing process and an edge bead rinsing process for removing excess coating.
In a typical photoresist process, a photoresist liquid is first coated onto a wafer by dispensing a small amount of the photoresist liquid at the center of a wafer that rotates at a high speed. Since only a thin coating of the photoresist material is normally required, the amount of the liquid photoresist material dispensed is relatively small. After the photoresist coating process is completed, a wafer can be imaged in a stepper to reproduce the circuits desired on the wafer.
The exposed wafer is then positioned in a second coating chamber wherein a liquid developer material is dispensed on the surface of the wafer by a technique similar to that used in dispensing the photoresist material. A perfectly centered dispensing nozzle for the developing liquid is important to the successful coating of a developing liquid on the wafer surface.
Another important criterion in carrying out a developing process for a photoresist layer is that during the developing process, the backside of the wafer must be protected from being contaminated by the developing liquid. A conventional developing liquid coating apparatus is shown in FIG. 1.
A conventional developing liquid coating apparatus 10 consists of several main components, first, a center vacuum chuck 12 which can be rotated at a high rotational speed by a shaft 14. Onto a top surface 16 of the vacuum chuck 12, a wafer 20 is positioned. The wafer 20 therefore rotates around a center axis 22 of shaft 14. During a developing liquid coating process, a dispensing nozzle (not shown) is centered with axis 22 and positioned very close to the top surface 24 of the wafer 20, i.e., at a distance of only a few millimeters apart. During the spin coating process, the backside 26 of the wafer 20 must be protected from contamination by the developing liquid. A drain cup 30 is provided for catching the excess, or spun-off developing liquid which includes a shield portion 32 and a base portion 34 that are usually formed integrally. The shield portion 32 collects droplets of the spun-off developing liquid such that it drains into cup 30 and is then taken away. The base portion 34 of the drain cup 30 is normally provided with backside rinse nozzles 38 which injects a rinse solution 42 toward the back surface 26 of the wafer 20 to further minimize the probability of contamination by the developing liquid sticking to the backside 26. Additionally, a knife edge ring 40 is utilized for providing a sharp edge 46 forming a seal with the backside 26 of the wafer to stop splattered developing liquid droplets from entering the compartment that houses the vacuum chuck. The knife edge ring 40 is attached to a top surface 36 of the base member 34 with mechanical means such that the gap "X" between the backside 26 of the wafer and the sharp edge, or seal 46 on the knife edge ring 40 can be suitably adjusted.
During a normal developing liquid coating process conducted on an eight inch wafer, a suitable distance of X is maintained at between about 0.5 mm and about 1 mm, and preferably between about 0.6 mm and about 0.8 mm. It should be noted that the drain cup 30 is stationary in relation to a spinning vacuum chuck 14 and wafer 20 carried thereon. Conventionally, the gap X must be adjusted after a preventive maintenance procedure is conducted, or when problem is observed in backside contamination by the developing liquid. FIG. 2 shows a conventional method for adjusting the gap between the backside 26 of the wafer 20 and the seal 46 on the knife edge ring 40. The adjustment is carried out by using a set of feeler gauges including the feeler gauge 50 shown in FIG. 2. After a suitable gap is first determined, e.g., 0.7 mm gap between the backside 26 and the seal 46, a 0.7 mm feeler gauge can be used for adjusting the position of the knife edge ring in relation to the backside of the wafer. Mechanical means such as adjusting screws are used for such purpose. For instance, for a eight inch wafer, a total of 6 adjusting screws are provided for mounting and adjusting the knife edge ring on the base member 34 of the drain cup 30. The screws are normally positioned in equal distance radially to the center axis 22 of the drain cup, or the vacuum chuck and circumferentially from each other.
The gap adjustment between the backside of the wafer and the knife edge ring is important such that the contamination of vacuum chuck by the developing liquid can be avoided. Furthermore, a suitable gap distance between the wafer backside and the knife edge ring at approximately 0.7.+-.0.1 mm must also be maintained. When the gap is too large, contamination of the vacuum chuck by the developing liquid can not be prevented. When the gap is too small, the backside rinse process can not be effectively carried out and furthermore, there may be a danger of scratching the backside of the wafer and causing severe damage.
The use of feeler gauges to measure and adjust the gap distance between the wafer backside and the knife edge ring is difficult and inadequate. Not only it is a time consuming process, the feeler gauge method is also very much operator dependent, i.e., skill-level dependent. For instance, feeler gauge adjustments made by different operators may be different and moreover, adjustments made by the same person at different times may also be different.
It is therefore an object of the present invention to provide a method and apparatus for measuring a gap distance between two opposing surfaces that does not have the drawbacks and shortcomings of the conventional methods and apparatus.
It is another object of the present invention to provide a method and apparatus for measuring a gap distance between two opposing surfaces by utilizing a dial gauge instead of feeler gauges.
It is a further object of the present invention to provide a method and apparatus for measuring a gap between two opposing surfaces by utilizing a dial gauge and a calibration block which has a thickness substantially the same as the gap distance to be measured.
It is another further object of the present invention to provide an apparatus for measuring a gap between two opposing surfaces by mounting a dial gauge on a mounting block and through extension arms such that the dial gauge may first be zeroed by using a calibration block and then be used to measure the deviation from zero of one of the opposing surfaces.
It is still another object of the present invention to provide an apparatus for measuring a gap distance between two opposing surfaces wherein a dial gauge is used which is capable of reading to an accuracy of at least 0.01 mm.
It is yet another object of the present invention to provide an apparatus for measuring a gap distance between two opposing surfaces further including the use of a calibration block which has a thickness substantially similar to the gap to be measured such that the dial gauge may first be zeroed on the block.
It is still another further object of the present invention to provide a method for measuring a gap distance between two opposing surfaces by first providing a measuring apparatus including a mounting block, a shaft extension, and a dial gauge and then zeroing the dial gauge with a calibration block which has a thickness that is substantially the same as the distance to be measured.
It is yet another further object of the present invention to provide a method for measuring a gap distance between two opposing surfaces of a wafer backside and a knife edge ring in a liquid coating chamber by first zeroing a dial gauge with a calibration block having a thickness substantially the same as the gap distance to be measured and then positioning the dial gauge on the surface to be measured such that any deviation from zero on the dial gauge can be read.